The present invention relates to a relatively inexpensive method and apparatus for coal desulfurization and de-ashing in which both the inorganic (pyrite) and organic sulfur is removed from coal along with removal of ash-forming minerals.
The cost of fuel oil as an energy source and its predicted depletion as well as the dependence on foreign sources makes the use of other type fossil fuels as fuel oil substitutes attractive. The abundance of coal in the United States and its accessibility suggests immediate direct substitution of coal for fuel oil where possible. An important factor limiting the substitution of coal for fuel oil is the effect of the by-products of coal burning on the ecology, particularly the sulfur by-products.
Untreated coal comprises organic carbonaceous material and inorganic minerals. Sulfur occurs in coal both in organic and inorganic forms. In the organic forms, the sulfur is chemically bonded into the hydrocarbon structure of the coal and generally cannot be removed by physical means such as magnetic separation. The inorganic forms of sulfur, generally occurs as pyrite, FeS.sub.2, but also includes other iron sulfur inorganics such as pyrrhotite Fe.sub.1-x S, and occurs as iron sulfide mineral inclusions in the coal and, therefore, can be removed magnetically. The relative proportions of organic and inorganic sulfur in coal vary with the source of the coal. In many coals from the Eastern half of the United States, the proportions are approximately equal.
Several techniques have been developed for removing sulfur from coal. Physical techniques, such as magnetic separation, are used to remove the inorganic sulfur. Chemical techniques, such as reacting coal with carbon disulfide, are used to remove the organic, as well as some of the inorganic sulfur. Thermo-chemical reactions can be caused by irradiating the coal aggregate with microwave energy to break bonds uniting organic coal components and sulfur contained in the coal aggregates (See Zavitsanos et al., U.S. Pat. No. 4,076,607). Chemical processes, such as reacting coal with carbon disulfide, are expensive since they generally involve the use of expensive chemicals. Using chemicals to remove the inorganic sulfur along with the organic sulfur is usually a much more expensive process than to use a physical method for inorganic desulfurization. Thermochemical processes using microwave heating along with numerous repeated processing steps with sodium hydroxide (caustic soda) are complex and still very expensive.
Purely physical processes for removal of the inorganic sulfur usually lose effectiveness when they encounter very small mineral particle sizes. This is particularly true for surface dependent techniques such as flotation or oil-water phase separation. These, as well as magnetic separation techniques do not address the problem of organic sulfur removal. In the application of the latter process, difficulty is encountered in separating weakly paramagnetic pyrite from the coal and the process is often ineffective in removing the non-pyritic ash-forming minerals.
For any physical separation, the coal must be crushed to liberate the mineral particles. Imperfect liberation leaves some coal associated with the minerals and even if these mixed particles are completely separated from the coal, some coal is lost. Excellent liberation is achieved by liquefying the coal to overcome the limitations of crushing and grinding but the liquefaction is accomplished at high temperature and pressure which; while it can alter the mineral magnetic properties to make magnetic separation more effective, nevertheless, it remains an expensive route to clean coal fuels. Such efforts have been largely abandoned in the United States because of cost. Magnetic methods of mineral removal from coal depend on the difference in the magnetic moment associated with mineral particles and that of coal. Coal is generally diamagnetic while some minerals are paramagnetic or have even stronger magnetic properties. As high grade coals have become scarce, coals used for steam generation often contain very fine mineral impurities making liberation difficult and expensive. In addition, magnetic properties are smaller for small particles. This can be overcome to some extent by altering those properties.
The approaches to magnetic coal cleaning can be divided into two categories relevant to the present invention: direct desulfurization and deashing, and separating coal minerals which have had pretreatment to enhance their magnetic properties.
The first, direct desulfurization has been carried out on crushed coal in water, oils, alcohol and in air or inert gas. The "direct" process depends on the difference between the magnetic properties of coal and those of its associated mineral impurities as found.
The second approach, enhancement of the magnetism of included coal minerals has been attempted by several methods: heating the whole coal, microwave irradiation to selectively heat the minerals, depositing iron selectively on the minerals from iron carbonyl (J. K. Kindig, The Magnex Process: Review and Current Status, Proceedings of the Conference on Industrial Applications of Magnetic Separation, Rindge, N. H. July 30-Aug. 4 1978, IEEE Publ. No. 78 CH1447-2 Mag.) and by adjustment of the atmosphere in an autoclave containing liquefied coal. Some of these efforts have had as their objective to improve desulfurization by HGMS and some to make other conventional magnetic separations more effective. (D. Kelland, "A Review of HGMS Methods of Coal Cleaning" IEEE Transactions on Magnetics Vol. MAG-18 No 3 May 1982)
As taught by Maxwell et.al in U.S. Pat. No. 4,466,362, issued Aug. 21, 1984, the success of magnetic separation is enhanced by the conversion of nonmagnetic, or weakly magnetic iron sulfides, to highly magnetic monoclinic pyrrhotite. Pyrrhotite is a nonstoichiometric compound with the approximate composition Fe.sub.0.9 S occurring in two crystalline forms: hexagonal pyrrhotite and monoclinic pyrrhotite. The monoclinic form is much more strongly magnetic than the hexagonal form but occurs only in a narrow range of compositions in the neighborhood of 47 atomic percent of iron. At 220.degree. C. conversion from the hexagonal to the monoclinic state is favored-and takes place rapidly; see "The Structure and Properties of Some Iron Sulphides", Reviews of Pure and Applied Chemistry, Vol. 20, p. 175, 1970.
In naturally occurring coal, sulfur appears mostly in the form of pyrites FeS.sub.2, which is only marginally paramagnetic. But, there are also small amounts of pyrrhotite present in the pyrite particles through the conversion of pyrite to pyrrhotite in nature and through heating during the subsequent grinding of the coal to form the necessary small particles for magnetic processing. These sulfides occur as interlocked particles differing in crystalline structure. For maximum efficiency in HGMS, the pyrrhotite inclusions in the pyrite should be in the more magnetic monoclinic state so that magnetic separation will be more complete.
Despite the intensive on-going investigations and experimentation in the field of coal desulfurization, a need still exists for a low cost desulfurization process which is effective in removing both the organic and inorganic sulfur.